Massive Gas Cloud Speeding Toward Collision With Milky Way

A giant cloud of hydrogen gas is speeding toward a collision with our
Milky Way Galaxy, and when it hits -- in less than 40 million years --
it may set off a spectacular burst of stellar fireworks.

GBT image of Smith's Cloud, which is headed toward a collision with the Milky Way.

Cedit: Bill Saxton, NRAO/AUI/NSF

Click image for high-resolution file (174 KB)

"The leading edge of this cloud is already interacting with gas
from our Galaxy," said Felix J. Lockman, of the National Radio
Astronomy Observatory (NRAO), leader of a team of astronomers who used
the National Science Foundation's Robert C. Byrd Green Bank Telescope
(GBT) to study the object. The scientists presented their findings to
the American Astronomical Society's meeting in Austin, Texas.

The cloud, called Smith's Cloud, after the astronomer who discovered
it in 1963, contains enough hydrogen to make a million stars like the
Sun. Eleven thousand light-years long and 2,500 light-years wide, it
is only 8,000 light-years from our Galaxy's disk. It is careening
toward our Galaxy at more than 150 miles per second, aimed to strike
the Milky Way's disk at an angle of about 45 degrees.

"This is most likely a gas cloud left over from the formation of the
Milky Way or gas stripped from a neighbor galaxy. When it hits, it
could set off a tremendous burst of star formation. Many of those
stars will be very massive, rushing through their lives quickly and
exploding as supernovae. Over a few million years, it'll look like a
celestial New Year's celebration, with huge firecrackers going off in
that region of the Galaxy," Lockman said.

When Smith's Cloud was first discovered, and for decades after, the
available images did not have enough detail to show whether the
cloud was part of the Milky Way, something being blown out of the
Milky Way, or something falling in.

Lockman and his colleagues used the GBT to make an extremely detailed
study of hydrogen in Smith's Cloud. Their observations included nearly
40,000 individual pointings of the giant telescope to cover the cloud
with unprecedented sensitivity and resolution. Smith's Cloud is about
15 degrees long in the sky, 30 times the width of the full moon.

"If you could see this cloud with your eyes, it would be a
very impressive sight in the night sky," Lockman said.
"From tip to tail it would cover almost as much sky as the
Orion constellation. But as far as we know it is made entirely of
gas -- no one has found a single star in it."

The detailed GBT study dramatically changed the astronomers'
understanding of the cloud. Its velocity shows that it is
falling into the Milky Way, not leaving it, and the new data
show that it is plowing up Milky Way gas before it as it falls.

"Its shape, somewhat similar to that of a comet, indicates
that it's already hitting gas in our Galaxy's outskirts," Lockman said.
"It is also feeling a tidal force from the gravity of the Milky Way and
may be in the process of being torn apart. Our Galaxy will get a rain
of gas from this cloud, then in about 20 to 40 million years, the cloud's
core will smash into the Milky Way's plane," Lockman explained.

Artist's conception of Smith's Cloud approaching, then colliding with, our own Milky Way Galaxy in approximately 40 million years. Sequence of images shows the approach and collsion. Detail: GBT image of Smith's Cloud.

The cloud will likely strike a region somewhat farther from the
Galactic center than our Solar System and about 90 degrees ahead of us
in the Milky Way disk. The collision may trigger a period of rapid
star formation fueled by the new gas and the shock from the
collision. Some theories say that the ring of bright stars near the
Sun, called Gould's Belt, was created by just such a collision event.

Lockman worked with Robert A. Benjamin and A.J. Heroux of the
University of Wisconsin-Whitewater and Glen I. Langston of NRAO.

The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement
by Associated Universities, Inc.